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ARS Home » Northeast Area » Kearneysville, West Virginia » Appalachian Fruit Research Laboratory » Innovative Fruit Production, Improvement, and Protection » Research » Publications at this Location » Publication #324945
Title: An apple rootstock overexpressing a peach CBF gene alters growth and flowering in the scion but does not impact cold hardiness or dormancy

Author
item Artlip, Timothy - Tim
item Wisniewski, Michael
item Norelli, John
item ARORA, RAJEEV - Iowa State University

Submitted to: Horticulture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/2016
Publication Date: 3/9/2016
Citation: Artlip, T.S., Wisniewski, M.E., Norelli, J.L., Arora, R. 2016. An apple rootstock overexpressing a peach CBF gene alters growth and flowering in the scion but does not impact cold hardiness or dormancy. Horticulture Research. DOI: 10.1038/hortres.2016.6.

Interpretive Summary: Due to 'global warming', many types of fruit crops undergo budbreak and flowering earlier and earlier in the spring and are then subsequently injured or killed when more seasonable, freezing weather returns. Over the past few years, there have been several spring frost events that have resulted in billions of dollars of lost profit to fruit growers and higher prices to consumers. Therefore, there is still a distinct need to develop fruit cultivars with improved hardiness and/or delayed dormancy. Previous USDA-ARS research demonstrated that a transgenic apple rootstock (T166) overexpressing a C-repeat Binding Factor (CBF) gene had increased levels of freezing tolerance, delayed spring budbreak, and reduced growth. The current study was conducted to determine if the demonstrated traits could be transferred to scion varieties of apple when they were grafted on the transgenic rootstock. Results indicated that dormancy and freezing tolerance of the scion, 'Royal Gala' (RG), was unaffected when it was grafted on T166 rootstock. Growth and flowering in the grafted scion, however, were reduced relative to the same cultivar grafted on non-transgenic M.26 rootstocks. Further studies are planned to determine whether or not individual branches grafted on a transgenic tree will exhibit the characteristics of the transgenic tree. In general, it appears that genetic modification of individual cultivars will be needed to improve freezing tolerance and regulate dormancy rather than these traits being transferred to the scion cultivar through transgenic rootstocks. The reduced growth exerted by the transgenic rootstock on scion cultivars, however, will be of interest to horticulturalists looking for new sources of dwarfing rootstocks.

Technical Abstract: The C-repeat Binding Factor (CBF) transcription factor is involved in responses to low temperature and water deficit in many plant species. Overexpression of CBF genes leads to enhanced freezing tolerance and growth inhibition in many species. The overexpression of a peach CBF (PpCBF1) gene in a transgenic line of own-rooted apple (Malus x domestica) M.26 rootstock (T166) trees was previously reported to have additional effects on the onset of dormancy and time of spring budbreak. In the current study, the commercial apple cultivar, 'Royal Gala' (RG), was grafted onto either non-transgenic M.26 rootstocks (RG/M.26) or transgenic M.26 (T166) rootstocks (RG/T166) and field-grown for three years. No PpCBF1 transcript was detected in the phloem or cambium of RG scions grafted on T166 rootstocks indicating that no graft transmission of transgene mRNA had occurred. In contrast to own-rooted T166 trees, no impact of PpCBF1 overexpression in T166 rootstocks was observed on the onset of dormancy, budbreak, or non-acclimated leaf cold hardiness in RG/T166 trees. Growth, however, as measured by stem caliper, current-year shoot extension, and overall height, was reduced in RG/T166 trees compared to RG/M.26 trees. Although flowering was evident in both RG/T166 and RG/M.26 trees in the second season, the number of trees in flower, the number of shoots bearing flowers, and the number of flower clusters per shoot was significantly higher in RG/M.26 trees than RG/T166 trees in both the second and third year after planting. Elevated levels of RGL (DELLA) genes were seen in RG/T166 trees and T166 trees, which may play a role in the reduced growth observed in these tree types. A model is presented indicating how CBF overexpression in a rootstock might influence juvenility and flower abundance in a grafted scion.